The present invention relates to an engine automatic control system for automotive vehicles, and more particularly to such engine automatic control systems as disclosed in U.S. Pat. Nos. 3,731,108; 3,872,316; and 3,949,236; wherein the engine is automatically stopped when the motion of the vehicle is stopped and is automatically driven in response to operation of a starting mechanism such as an accelerator pedal or clutch pedal for effecting start of the vehicle, these operations being carried out without operating an ignition switch of the engine.
In such engine automatic control systems as described above, the motion of the vehicle is detected by a speed sensor assembly such that an engine control circuit for the system is maintained in its activated condition in dependence upon change of the level of speed pulses from the sensor assembly to maintain the engine in its operative condition after start of the vehicle. For example, the speed sensor assembly includes a disk of permanent magnet and a reed-switch in magnetic coupling with projections formed on the outer periphery of the disk. The disk is fixed to a rotary member such as a speedometer cable which is arranged to be driven by an output shaft of a power transmission of the vehicle. When the disk is rotated in driving operation of the speedometer cable, the reed-switch acts to detect each projection of the disk so as to produce a series of speed pulses corresponding with the actual speed of the vehicle. In operation of the above control systems, it is, therefore, presumable that if the speed sensor assembly is out of order due to disconnection of the speedometer cable, damage of the reed-switch or the like prior to start of the vehicle, the engine is automatically stopped resulting in unexpected trouble of the operator.